It's trivial to run R5RS in DrRacket, for example, or other Scheme implementations as well. So it shouldn't really be a problem if R7RS becomes bloated, as you can stay on R5.
Well, note that this is about R7RS-large; R7RS-small is "a 'small' language, suitable for educators, researchers, and users of embedded languages, focused on R5RS compatibility" (http://trac.sacrideo.us/wg/wiki/R7RSHomePage see also http://www.scheme-reports.org/), its spirit is R5RS cleaned up and with a library feature to allow R7RS-large to be cleanly added.